through the thin crust. Great floods of dark basaltic lava spilled onto the sur-
face,giving the Moon a landscape of giant craters and flat lava plains (Fig. 9)
called mariafrom the Latin word for “seas.”
The Moon became gravitationally locked onto its mother planet, rotating
at the same rate as its orbital period, causing one side always to face Earth. Many
moons around other planets share similar characteristics with the Earth’s moon,
suggesting they were created in much the same manner. Since Earth’s sister
planet Venus formed under similar circumstances and is so much like our planet,
the absence of a Venusian moon is quite curious. It might have crashed into its
mother planet or escaped into orbit around the Sun. Perhaps Mercury, which is
about the same size as the Earth’s moon, was once a moon of Venus.
The newly formed moon circled just 14,000 miles above Earth, racing
around the planet every two hours. It orbited so close to Earth it filled much
of the sky. The Moon’s nearness also explains why the length of day was so
much shorter.The early Earth spun faster on its axis than it does today. As the
planet slowed its rotation due to drag forces caused by the tides (making daysFigure 7The giant
impact hypothesis of lunar
formation envisions a
Mars-sized planetesimal
(A)colliding with the
protoearth (B)resulting in
a gigantic explosion and
the jetting outward of both
planetesimal and
protoearth material into
orbit around the planet.
(C)A protomoon begins to
form from a prelunar disk,
and matter accretes to form
the Moon (D).PLANET EARTHAB
CD